Correspondence to Dr Anita E Heywood, School of Public Health and Community Medicine, UNSW Medicine, The University of New South Wales, Level
2, Samuels Building, Sydney, NSW 2052, Australia; a.heywood{at}unsw.edu.au

Received 20 May 2013

Revised 15 July 2013

Accepted 19 July 2013

Published Online First 21 August 2013

Abstract

Background Abundant, indirect epidemiological evidence indicates that influenza contributes to all-cause mortality and cardiovascular
hospitalisations with studies showing increases in acute myocardial infarction (AMI) and death during the influenza season.

Objective To investigate whether influenza is a significant and unrecognised underlying precipitant of AMI.

Results Of 559 participants, 34/275 (12.4%) cases and 19/284 (6.7%) controls had influenza (OR 1.97, 95% CI 1.09 to 3.54); half were
vaccinated. None were recognised as having influenza during their clinical encounter. After adjustment, influenza infection
was no longer a significant predictor of recent AMI. However, influenza vaccination was significantly protective (OR 0.55,
95% CI 0.35 to 0.85), with a vaccine effectiveness of 45% (95% CI 15% to 65%).

Conclusions Recent influenza infection was an unrecognised comorbidity in almost 10% of hospital patients. Influenza did not predict
AMI, but vaccination was significantly protective but underused. The potential population health impact of influenza vaccination,
particularly in the age group 50–64 years, who are at risk for AMI but not targeted for vaccination, should be further explored.
Our data should inform vaccination policy and cardiologists should be aware of missed opportunities to vaccinate individuals
with ischaemic heart disease against influenza.

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